A dynamic bearing supports an axial member in a non-contact manner by a fluid dynamic pressure generated in a gap in the bearing. A bearing device using such a dynamic bearing (hereinafter, referred to as dynamic bearing device) is generally classified into a contact type and a non-contact type. In the contact type, a radial bearing part is formed by a dynamic bearing while a thrust bearing part is formed by a pivot bearing. In the non-contact type, both the radial bearing part and the thrust bearing part are formed by dynamic bearings. Those two types of bearing devices are used to suit their application.
An exemplary dynamic bearing device of a non-contact type is disclosed in Japanese Patent Laid-Open Publication No. 2000-291648 proposed by the applicant of the present application. This dynamic bearing device has a T-shaped axial member including an axial part and a flange part that are integrated as one unit in order to reduce a cost and improve precision.
Conventionally, in many cases, an axial member with a flange part is formed from stainless steel in consideration of abrasion resistance, while a housing is formed from brass in consideration of processability. In these cases, the amount of thermal expansion in an axial direction when the temperature increases is larger in the housing formed of brass than in the flange part formed of stainless steel because brass has a larger linear expansion coefficient than that of stainless steel. In a general dynamic bearing device, because of decrease of viscosity of fluid (oil) at high temperatures, decrease of bearing rigidity, especially in a thrust direction is a problem. This bearing rigidity in the thrust direction further decreases when the linear expansion coefficient of the housing is larger than that of the flange part. This is because the width of the gap in the thrust bearing becomes larger at high temperatures. On the other hand, at low temperatures, a motor torque is increased by increase of viscosity of fluid. The difference between the above linear expansion coefficients acts to increase the motor torque. As described above, the conventional structure has a drawback that the difference of the amount of thermal expansion between the flange part and the housing acts to aid the aforementioned problems both at high temperatures and at low temperatures.